Potentiometric slide elements for so-called "dry assays" are well-known, and are described, for example, in U.S. Pat. Nos. 4,184,936 and 4,273,639. Such known elements feature a pair of identical ion-selective-electrodes, hereinafter "ISE's", coupled together with an ion bridge that is generally fibrous in nature. One of the two ISE's is contacted with patient sample liquid, and the other with a reference fluid. The ISE's function to provide a differential charge, depending on the relative ionic concentrations of the analyte, for a differential analysis. The ISE's are exposed at opposite surfaces, so that their ion-selective membranes are exposed to the sample liquid or reference liquid, and their metal layers are exposed to the contacts of an electrometer.
Because of the format of the ISE's and the connecting bridge, the overall slide element has been rectangular in configuration. Although such a configuration has worked well, it is constraining in that it does not lend itself to, e.g., a circular configuration. Indeed, prior to this invention it has been difficult to provide a circular potentiometric slide element using paired ISE's and a connecting ion bridge, precisely because such ISE's and a bridge have not been so configurable.
Circular potentiometric slide elements, on the other hand, become essential if future analyzers are to be converted to slide elements that are circular for all assays (both colorimetric and potentiometric). Unlike those using rectangular frames, slide elements constructed from round frames have no special orientation in an analyzer. This is both a benefit and a possible hindrance. The benefit is that the slide elements can be rotated into any particular station without regard to any particular orientation of a side edge. The possible hindrance is that if there is any feature of the slide element that lacks axial symmetry, it is difficult to position a round slide element to take that asymmetry into account.
Colorimetric slide elements easily lend themselves to a round format; potentiometric elements do not.
Yet another disadvantage of the rectangular ISE slide elements is that the sample deposit must occur off-center. This is contrary to the center depositing of sample that occurs for colorimetric slide elements, necessitating therefore a complicated alteration of the dispenser tip location when the type of slide element changes. For ease in dispensing, it is preferred that all sample dispensing occur at the center of the slide element.